function onRefreshClick(field_id_prefix, backend){
  var d = new Date();
  timestamp = d.getTime();
  $.post(backend, 
         {time: timestamp}, 
         function(json) {
           refreshCaptcha(json, field_id_prefix); 
         }
  );
}

var isdone = false;

function refreshCaptcha(json, field_id_prefix) { 
  eval("var data = " + json);
  var images = document.getElementsByTagName('IMG');
  var inputs = document.getElementsByTagName('INPUT');
  inputs[field_id_prefix+'_0'].value = data['captcha_id'];
  inputs[field_id_prefix+'_1'].value = data['src'];
  images[field_id_prefix+'_C'].src = data['src'];
  isdone = true;
}